For much of the twentieth century, chromite ore was processed at various locations in the United States, to manufacture chromium and related materials. Processing the chromite ore created large amounts of chromite ore processing residue (COPR). Millions of tons of COPR were then placed into the ground, often at or near the processing locations. These sites, which are now contaminated with COPR, are in or near densely populated urban and waterfront areas in United States. There are similarly contaminated sites in Europe, Japan, and other countries.
COPR is similar in texture to coarse gravel. It is formed as solid nodules or pellets generally ¼ to ½ inch in diameter, as a waste product from ore processing. These pellets were often used like gravel, as grading and fill material, and also in construction of residential, commercial and industrial buildings. COPR was also used in roadbeds and pipeline trenches. Consequently, some COPR deposits may extend for thousands of feet under dense urban development. In addition, in many of these locations, the COPR is below the ground water table. In some locations, the COPR is mixed with other materials, such as soil, sand, etc.
COPR is a strong alkaline or caustic material. It typically has a pH of about 11-12. COPR typically also contains %1-%30 of hexavalent chromium, having the chemical symbol Cr(VI). Cr(VI) is toxic to humans. It can be absorbed into the body via the skin, mouth or via inhalation. It is known to cause cancer and genetic mutations. Consequently, COPR presents serious environmental and public health hazards.
Cr(VI) is also present in other types of contaminated sites, dissolved in ground water. The Cr(VI) may be the result of releases from metals plating operations, from the application of Cr(IV) corrosion inhibitors, and from landfills or other disposal sites.
At COPR contaminated sites, the chromium is present in the solid particles as well as in the ground water in the pores or spaces between the COPR particles or pellets. Since Cr(VI) is soluble in water, if the pore water is removed, the hexavalent chromium is replaced by a slow diffusion or leaching of additional hexavalent chromium from within the particles. As a result, pump and treat or soil washing is ineffective or at least impractical for treatment of COPR sites.
Cr(VI) in pore water can be converted to trivalent chromium, which has the chemical symbol Cr(III), using remediating chemical compounds. These compounds include soluble ferrous iron salts, such as ferrous sulfate or ferrous chloride, or other similar remediating compounds. Cr(III) is insoluble and relatively non-toxic. Accordingly, if the Cr(VI) could be substantially completely converted to Cr(III), the COPR at many sites could then be safely left in the ground. However, with these chemical remediation methods, the soluble remediating compounds tend to be washed away by ground water movement relatively quickly. Consequently, the conversion process expectedly does not last long enough to clean up the site.
Other in-situ clean up processes use biological reduction of the Cr(VI), with or without use of other remediating materials. In biological clean up techniques, organic materials containing bacteria and nutrients are mixed into the COPR contaminated soil. However, in general, these types of biological reduction techniques require a pH conducive for growth of bacteria, typically about 6.5 to 9.5. Consequently, biological techniques have required adding large amounts of acid into the contaminated site, to lower the pH to a level acceptable for growth of bacteria. The acid causes destruction of the COPR particle structure. This can make future handling of the COPR more difficult. The acid also generates large volumes of carbon dioxide gas. In addition, placing large amounts of acid into the ground can damage structures on or in the ground. The disadvantages of the need for this use of acids has largely prevented effective use of biological remediation techniques on COPR.
In view of these problems, plans for permanent clean up of COPR sites have largely contemplated excavation and removal of the COPR material. This can require demolition, in-fill, and reconstruction of buildings on the contaminated sites. Moreover, the excavated material must still be remediated off site to convert the Cr(IV) to Cr(III), before it can be placed in landfill or other final disposal site. The costs, disruption, and delays associated with excavation and removal of the contaminated material can of course be enormous. Treating sites having dissolved Cr(IV) presents similar problems. Accordingly, improved methods for cleaning up COPR and dissolved Cr(IV) contaminated sites are needed.